Balancing your RF levels to get the best RF performance

Wrote this in a PM to another member, but I realized it might help others.

This short tutorial is for the 900 radios, but applies to all frequencies:

Balance the SM’s
Look at the SM with the lowest power level on the AP sessions page that has the lowest number of re-reg’s and make that your low limit. Then try to get the power level on each SM adjusted to that they are as close to that as possible.

If the SM with lowest level = -76dB, the highest SM should be -66dB. Any SM’s above that should be adjusted down in small increments until they all fall within the 10dB window - ideally right between the high and low limits (Remember you are looking at the AP sessions page, not the SM status page.) You may find that your window needs to be higher, like -70 to -60, this tells you that your noise floor is somewhere right below -70 and any SM’s below -70 probably will not stay registered. The only solution for those is more gain, better LOS, or more power. Once you have adjusted the power levels, you may find that SM’s that were marginal previously start registering.

Look at the spectrum:
Once the SM’s are all balanced, turn off all AP’s and flip them all to SM. Do a spectrum analysis on each taking a screen shot of each - wait about 3-4 minutes before taking the screen shot, this will reveal any spurious RF level peaks. If you see any levels that are significantly higher (more than about 6dB) than the high limit you set for your SM’s you know there is a source of interference and you will need to avoid that frequency.

Another thing that might give you some info would be to grab an SM with a 13dB yagi, go to the tower and turn off all of the AP’s. Put the SM in spectrum analyser mode with the SM’s refresh rate at 3sec. Very slowly sweep the yagi around 360degrees and see what pops up on the analyzer page. You may see a significant source of energy and be looking right at a tower nearby.

Then you can go to that tower and see if you can find out what/who the source is, and if they are able and/or willing to move to another frequency. You might find that the city or county or other entity has recently installed a 900MHz link right on one of your channels! Happens all the time. People are usually willing to work on frequency co-ordination as it helps them as much as it helps you.

Hope this helps.

Excelent document Jerry, thanks.

Why is it important to balance RF levels? What happens if you don’t? (I have several AP’s that have variations greater than this.) And why 10 dB, is this due to the FM capture effect?

The best analogy I have is this:

You have someone standing 10’ from you shining a 10watt flashlight (SM) into your eyes (AP).

There is another person standing 100’ from you shinging a 10w flashlight (SM) toward your eyes (AP).

All you can see is the closer flashlight because your eyes are desensitized to the weaker light.

Dim the closer flashlight (lower SM transmit power) so that both flashlights are equally bright in your eyes and you will be able to both with no problems.

The reason for 10dB is that it’s pretty difficult to get them all exactly the same (although that would be ideal). A 10dB window allows for variances and still seems to perform well.

Pretty good analogy there =P